When Less Is More: Low-Silver Lead-Free Solders

Posted 09/15/2011 by $username

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Tin-silver-copper solders (e.g., SAC305 – Sn96.3Ag3.0Cu0.5) have largely taken over electronics assembly. Their wetting and reliability performance are adequate, and their liquidis temperature is low enough that conventional wave and reflow soldering equipment can be used.

But the high cost of silver requires an examination of price vs. performance. At 2010 bulk metal prices, tin-lead solder costs about $11/kg, while the cost of SAC305 solder is in the $38/kg range.

To help resolve this new price-performance tradeoff, Alpha has undertaken a wide-ranging study of the mechanical and soldering performance attributes of low-silver lead-free solders. Some of the results include:

  • As silver content in SAC solder increases, thermal fatigue resistance improves, but drop shock reliability falls, especially in solder alloys having more than about 1% silver.
  • The addition of small (~0.1%) amounts of bismuth to low-silver SnAgCu solder alloys refine the microstructure, leading to large improvements in thermal fatigue resistance.
  • Silver levels up to approximately 1% in SAC solder greatly influence the alloy’s wetting behavior. Higher amounts have little additional effect.

As a result of these studies, Alpha has made available a series of low silver solder alloys engineered for providing just the right level of soldering and reliability performance for any type of electronics assembly being made. These alloys are SACX Plus 0107, SACX Plus 0307 and SACX Plus 0807.

What experiences have you had in assessing low-Ag, Sn-Ag-Cu alloys?